THE OPTIMIZATION OF WEDGE FILTERS IN RADIOTHERAPY OF THE PROSTATE

A treatment plan optimisation algorithm has been applied to 12 patient s with early prostate cancer in order to determine the optimum beam-we ights and wedge angles for a standard conformal three-field treatment technique. The optimisation algorithm was based on fast-simulated-anne aling using a cost function designed to achieve a uniform dose in the planning-target-volume (PTV) and to minimise the integral doses to the organs-at-risk. The algorithm has been applied to standard conformal three-field plans created by an experienced human planner, and run in three PLAN MODES: (1) where the wedge angles were fixed by the human p lanner and only the beam-weights were optimised; (2) where both the we dge angles and beam-weights were optimised; and (3) where both the wed ge angles and beam-weights were optimised and a non-uniform dose was p rescribed to the PTV. In the latter PLAN MODE, a uniform 100% dose was prescribed to all of the PTV except for that region that overlaps wit h the rectum where a lower (e.g., 90%) dose was prescribed. The result ing optimised plans have been compared with those of the human planner who found beam-weights by conventional forward planning techniques. P lans were compared on the basis of dose statistics, normal-tissue-comp lication-probability (NTCP) and tumour-control-probability (TCP). The results of the comparison showed that all three PLAN MODES produced pl ans with slightly higher TCP for the same rectal NTCP, than the human planner. The best results were observed for PLAN MODE 3, where an aver age increase in TCP of 0.73% (+/-0.20, 95% confidence interval) was pr edicted by the biological models. This increase arises from a benefici al dose gradient which is produced across the tumour. Although the TCP gain is small it comes with no increase in treatment complexity, and could translate into increased cures given the large numbers of patien ts being referred. A study of the beam-weights and wedge angles chosen by the optimisation algorithm revealed significant inter-patient vari ability the implications of which are examined. Probably the most sign ificant benefit of the algorithm is the time saved (about a factor of 10) in computing optimised beam-weights and wedge angles for this simp le plan.